1. Technical Field
The present invention relates to a pressure responsive device such as an electret condenser microphone or a pressure sensor for use in a cellular phone or the like, and further to a method of manufacturing a semiconductor substrate for use in such a pressure responsive device.
2. Background Art
FIG. 7 is a sectional view showing a conventional electret condenser microphone for use in a cellular phone or the like. In the drawing, reference numeral 20 is a printed board on which a junction FET (hereinafter referred to as J-FET) 21 is mounted, and numeral 22 is a back plate. Numeral 23 is an electret membrane semi-permanently charged with an electrical charge (Q) by irradiating a polymer, e.g., polypropylene with an electronic beam. Numeral 24 is a spacer made of a plastic, and numeral 25 is a vibrating membrane disposed above the electret membrane 23 via the spacer 24 and coated with a surface electrode made of aluminum. This vibrating membrane 25 is opposite to the electret membrane 23 and the back plate 22 therebelow via a space, and forms a capacitor between these electret membrane 23 and back plate 22 and the vibrating membrane 25. Furthermore, numeral 26 is a retaining rubber for fixing the vibrating membrane 25. Numeral 27 is a holder for holding the back plate 22 and the electret membrane 23. Numeral 28 is a capsule including a vent hole 29, and numeral 30 is a cloth covering the vent hole 29.
In the conventional electret condenser microphone, the capacitor is constructed of the back plate 22, the electret membrane 23 and the vibrating membrane 25 having the surface electrode. When a sound pressure such as a sound or voice is transferred through the vent hole 29 of the capsule 28, the vibrating membrane 25 is vibrated by this sound pressure thereby a capacity (c) of the capacitor being varied. Since an electrical charge (Q) is constant, variation in a voltage (V) is produced on the basis of Q=CV. Applying the voltage variation to a gate electrode of J-FET 21 causes variation in drain current, which is detected in the form of voltage signal.
Since an electret condenser microphone is used for a take-along terminal, e.g., a cellular phone, further thinning and miniaturization thereof have been desired. In the conventional construction of above construction, however, the printed board 20, J-FET 21, the holder 27 and the like are used resulting in a large number of parts. Therefore thinning and miniaturization of the electret condenser microphone were difficult. Moreover in the mentioned conventional construction, a problem exists in that S/N ratio is lowered as being thin and small-sized, eventually resulting in worse performance.
The present invention was made in order to solve the above-discussed problems, and has an object of providing a pressure responsive device capable of achieving thinning or miniaturization thereof while maintaining a high performance. The invention also provides a method of manufacturing a semiconductor substrate for use therein.
A pressure responsive device according to the invention comprises: a package including a storage chamber in an interior thereof; means for introducing an outside pressure into the storage chamber; a semiconductor substrate placed in the storage chamber, provided with a pair of main surfaces opposite to each other; a first electrode disposed on one of the main surfaces; spacer means positioned in a peripheral portion of said first electrode and disposed on one of the main surfaces of the semiconductor substrate, the spacer means is composed of polyimide and having a supporting surface; and a vibrating membrane of which peripheral portion is supported by the supporting surface of the spacer means, the vibrating membrane includes a second electrode which is opposite to the first electrode via a space and forms a capacitor together with the first electrode.
In the pressure responsive device according to the invention, it is preferable that a flattening membrane for flattening the supporting surface of the spacer means is provided on the supporting surface of the spacer means.
In the pressure responsive device according to the invention, it is preferable that a silicon nitride membrane is used as the flattening membrane.
In the pressure responsive device according to the invention, it is preferable that the semiconductor substrate includes a conversion circuit for converting variation in capacity of the capacitor due to vibration in the vibrating electrode membrane into a voltage signal.
In the pressure responsive device according to the invention, it is preferable that the spacer means includes plurality of spacers forming communication clearances to the storage chamber therebetween.
In the pressure responsive device according to the invention, it is preferable that a thickness of the spacer means is in the range of 10 to 20 xcexcm in a direction that the first electrode and second electrode are opposing to each other.
In the pressure responsive device according to the invention, it is preferable that an electret membrane formed by electrically charging a polymer coated with the second electrode is used as the vibrating membrane.
A method of manufacturing a semiconductor substrate according to the invention, the semiconductor substrate being used in the pressure responsive device and having spacer means composed of polyimide on one of a pair of main surfaces opposite to each other, comprises the steps of: applying polyimide onto a semiconductor substrate and setting the polyimide membrane at 300xc2x0 C. to 370xc2x0 C.; forming a silicon nitride membrane on the polyimide membrane; applying a resist onto the silicon nitride membrane and forming a resist pattern by a photomechanical process; etching the silicon nitride membrane using said resist pattern and as a mask; etching said polyimide membrane for forming the spacer means using said resist pattern and said silicon nitride pattern as a masks, and removing the resist after etching said polyimide membrane.
In the pressure responsive device of above construction according to the invention, spacer means composed of polyimide is disposed onto the semiconductor substrate having the first electrode on one of the main surfaces. Further, the peripheral portion of the vibrating membrane is supported by the supporting surface of the spacer means thereby forming a capacitor comprised of the first electrode/the space (air)/the second electrode. As a result, the number of parts becomes smaller than that in the conventional device of the same type and moreover each part is small-sized, and consequently it is possible to achieve thinning and miniaturization of the device while maintaining a high performance.
Further, in the mentioned pressure responsive device wherein the flattening membrane is provided on the supporting surface of the spacer means to flatten the supporting surface, or wherein the silicon nitride membrane is utilized as this flattening membrane, it is possible to prevent irregularity in thickness of the polyimide spacer means in each device. Consequently, irregularity in performance of each device is also suppressed and thus a highly reliable pressure responsive device can be obtained. Furthermore, in the mentioned pressure responsive device wherein the thickness of the spacer means is 10 to 20 xcexcm in the direction that the first electrode and second electrode are opposing to each other, the spacer means can be made to effectively respond to even slight and small vibrations of the vibrating membrane. Additionally, in the mentioned pressure responsive device wherein the semiconductor substrate is provided with the conversion circuit for converting variation in capacity of the capacitor into electric signals, the electric signals can be easily fetched. Further, in the mentioned pressure responsive device wherein the spacer means includes plurality of spacers forming communication clearances to the storage chamber, there is an advantage of achieving more free vibrations of the vibrating membrane. In the mentioned pressure responsive membrane wherein an electret membrane formed by electrically charging a polymer coated with the second electrode is used as the vibrating membrane, vibrations of the vibrating membrane can be effectively converted into variation in capacity of the capacitor.
Furthermore, in the mentioned method of manufacturing the semiconductor substrate according to the invention, the semiconductor substrate having the spacer means composed of polyimide can be easily manufactured on one of the main surfaces in the same manner as the conventionally known method of manufacturing a semiconductor apparatus. Therefore, it is possible to manufacture economically and in large quantity the semiconductor substrate used in the pressure responsive device.